Method for etching thin film materials by direct cathodic back sputtering

ABSTRACT

Thin layers of material including dielectric films are etched or cleaned by placing them in a low pressure gas ambient, forming a plasma in the ambient, and establishing a periodic voltage between the layers and the plasma. One important application of the process is the formation of metal silicide contacts through small windows in a dielectric layer protecting the silicon surface. In one application, the metal is sputtered onto the exposed silicon at the same time that the surface is subjected to ion bombardment. The sputtering and etching rates are adjusted so that some of the sputtered metal reacts with the silicon upon impact and the unreacted metal is etched away.

United States Patent Byrnes, Jr. et al.

[451 May 9, 1972 [72] Inventors: Peter A. Byrnes, Jr., BridgewaterTownship, Somerset County; Martin P. Lepselter, New Providence, both ofNJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: Aug. 11, 1969 [21] Appl. No.: 848,935

Related [1.5. Application Data [63] Continuation-in-part of Ser. No.607,203, Jan. 4,

[52] US. Cl ..204/192 [51] Int. Cl C23c 15/00 [58] Field of Search..204/192 [56] References Cited UNITED STATES PATENTS 3,479,269 11/1969Byrnes et al. ..204/298 3,287,612 11/1966 Lepselter ..317/235 3,451,9126/1969 DHeurle et al. ..204/192 Laegreid et al. ..204/192 3,021,2712/1962 Wehner ..204/192 OTHER PUBLICATIONS Davidse, Theory &,Practive ofRF Sputtering Vacuum Vol. 17, No. 3, l966),pg. 145

Maissel et al. Thin Films Deposited by Bias Sputtering J. of App. Phy.,1965 Primary Examiner.lohn H. Mack Assistant ExaminerSidney S. KanterAttorney-R. J. Guenther and Arthur J. Torsiglieri [57] 1 ABSTRACT Thinlayers of material including dielectric films are etched or cleaned byplacing them in a low pressure gas ambient, forming a plasma in theambient, and establishing a periodic voltage between the layers and theplasma. One important application of the process is the formation ofmetal silicide contacts through small windows in a dielectric layerprotecting the silicon surface. In one application, the metal issputtered onto the exposed silicon at the same time that the surface issubjected to ion bombardment. The sputtering and etching rates areadjusted so that some of the sputtered metal reacts with the siliconupon impact and the unreacted metal is etched away.

3 Claims, 8 Drawing Figures PATENTEDMAY 9 I972 3, 51,747

SHEU 1 [IF 4 WVENTORS" M. P. LEPSELTER ATTORNFV P. ,4 BYRNES,JR.

SHEET 2 OF 4 mox.

FIG. 4

KNEE 2 KNEE 1 max PATENTEDMAY 9 I972 HEET 4 [1F 4 FIG. 7

TO VACUUM SYSTEM METHOD FOR ETCHING THIN FILM MATERIALS BY DIRECTCATI-IODIC BACK SPUTTERING CROSS REFERENCES TO RELATED APPLICATIONS Thisapplication is a continuation-in-part of the copending application, Ser.No. 607,203, filed Jan. 4, 1966 by P. A. Bymes, Jr. and M. P. Lepselter.

BACKGROUND OF THE INVENTION This invention relates to cleaning oretching by cathodic back sputtering. More specifically, it concerns amethod for cleaning or etching which utilizes a periodic voltage toextract bombarding ions from a plasma.

Since the dimensions of microelectronic devices and circuits aremeasured in thousandths of an inch, precise methods of etching andcleaning are of considerable importance in their fabrication. Varioustechniques, including chemical etching and indirect back sputtering havebeen adapted for use in microelectronic fabrication processes, but thesemethods have important limitations.

Chemical etching, such as is described by Schlabach and Rider in Printedand Integrated Circuitry (1963) at p. 83 et seq. is one commonly usedetching or cleaning method. How,- ever, there are at least threedifficulties associated with this approach. First, because the erodingaction proceeds at a different rate as the etching depth increases,chemical etching generally produces cross sections that are eitherundercut or slop-sided rather than rectilinear. Second, since differentmaterials react differently to the same etchant, chemical etchingbecomes a time-consuming, multi-step process when it is necessary toetch through several layers of different materials; and, third, chemicaletching is not useful with certain materials, such as iridium andrhodium, because of their high resistivity to chemical action.

A second approach to etching and cleaning, which is of particular valuein the fabrication of microelectronic circuits and devices, is positiveion bombardment. When positive ions, moving at high velocities, collidewith the surface of a work piece, they remove material from it. Thus,for example, the workpiece may be etched or cleaned by placing it on acathode in a low pressure noble gaseous ambient and applying a highconstant voltage between the cathode and an anode. Ions, which areformed by collisions between electrons accelerated from the cathode andnoble gas atoms, are accelerated toward the cathode where they bombardthe workpiece surface.

Present methods utilizing ion bombardment are, however, of limitedutility in the etching or cleaning of workpieces containing thin filmsof dielectric materials or other materials which cannot be subjected tohigh, constant voltages without damage. Typically, the voltages requiredto form ions and obtain etching are in excess of the breakdown voltageof thin dielectric films. And while the problem of breakdown may beovercome by placing an insulating layer between the workpiece and thecathode, (see M. P. Lepselter, U.S. Pat. No. 3,271,286, dated Sept. 6,1966) the efficiency ofthe process is greatly reduced by the presence ofan insulator because the electric field must be bent around it. As aresult, the etching is produced by bombardment of stray ions from thesurrounding field rather than by ions attracted directly to theworkpiece, and is less satisfactory.

The method for etching or cleaning, in accordance with the presentinvention, comprises, in brief, the steps of placing the workpiece in alow pressure noble gas ambient, forming a plasma in the ambient, andeffecting the bombardment of the surface by ions drawn from the plasmaby the establishment of a periodic voltage between the workpiece and theplasma. This method is particularly useful in the subsequent formationof an atomically clean bond between a surface of the workpiece and someother material.

When it is desired to deposit a layer of material on the surface of aworkpiece, the formation 'of an atomically clean bond is importantbecause of greater adherence and greatly LII increased uniformity in itselectrical properties. One difficulty typically encountered indepositing a layer of material on a metal or semiconductor workpiecesurface is the formation of an unwanted, thin, oxide-like surface layeron the workpiece surface prior to deposition. Typically, such a surfacelayer forms immediately after cleaning when the substrate is exposed toair, and forms even at moderate vacuums so low as 10 torr. The presenceof the surface layer prevents uniform reaction between the depositedmaterial and the workpiece because the reaction must take place throughrandomly distributed voids in the surface layer. But when theintervening surface layer is eliminated, more uniform depositions may beobtained. The result is to produce more uniform ohmic and rectifyingcontacts. (See Kahng & Lepselter, Planar Epitaxial Silicon SchottkyBarrier Diodes, Bell System Technical Journal 44:1525, 1965) While theadvantages of atomically clean bonds are known, previously devisedmethods of forming them are of only limited utility in the fabricationof microelectronic devices. One method of forming such a bond iscleaving a sample of material in a vacuum and depositing a layer ofmaterial on the freshly cleaved surface. It is, however, impractical tocleave the surface of a workpiece containing layers of material as thinas those typically encountered in microelectronics. A second method isvacuum heating a sample of material to a very high temperature and thendepositing a layer of material on it. A difficulty with this approach,however, is that the intense heat pits oxide layers and destroys theproperties of delicate junctions.

In accordance with an important use of the invention, an atomicallyclean bond is formed by cleaning the workpiece surface utilizing ionicbombardment and then depositing a layer of material upon the freshlycleaned surface.

The invention may now be described in greater detail by reference to theaccompanying drawings wherein:

FIG. 1 is a cross section of a typical workpiece to be etched or cleanedin accordance with the invention;

FIGS. 2, 5, 6 and 8 show various forms of apparatus which may be usedfor the practice of the invention;

FIG. 3 is a graph illustrating features of a preferred voltage waveformsuitable for application between the workpiece and the plasma; I

FIG. 4 is a graph of the ion-induced etching of a typical material as afunction of the voltage between the workpiece and the plasma; and

FIG. 7 is a typical workpiece upon which an atomically clean bond is tobe formed in accordance with the invention.

Similar reference characters are applied to similar elements throughoutall the drawings.

In FIG. 1 is shown a cross section of a typical workpiece. Typically, itcomprises a relatively thick substrate 10 such as, for example, asilicon wafer, supporting a thin film of material 1 l. The thin filmtypically comprises several layers which have been deposited insuccession upon the surface. These layers may be dielectrics,semiconductors, conductors or even ferrites. Also shown is a maskinglayer 12 which may be formed by known techniques, such as the well-knownphoto-resist method, to protect areas of the surface which are not to beetched. In typical applications the thin film 11 is a few tenthousandthsof an inch thick or smaller with the thickness of the individual layersbeing a few hundred-thousandths of an inch.

Reference is now made to FIG. 2 which is a schematic illustration ofapparatus used to practice the invented method.

The workpiece, along with an electron-emitting filament or cathode 20and an anode 21, is placed within a vacuum chamber 22. In one typicalarrangement the filament was placed approximately five inches from theanode and the workpiece was positioned two inches from thefilament-anode line. The chamber is also provided with apparatus (notshown) for evacuating it and injecting a suitable gaseous ambient intoit. Since a plasma 23 is to be formed between the filament and theanode, structure for containing a plasma and, incidentally,

for supporting the workpiece in proper relationship to the plasma, isadvantageously provided. In the apparatus of FIG. 2, a quartz container24, having suitably located apertures 25, 26, and 27 for the workpiece,filament and anode, respectively, is provided for this purpose.Additionally, apparatus for establishing a longitudinal magnetic fieldalong the filamentanode direction, such as, for example, ring magnets28, are provided in order to controlthe plasma. Terminals extendingthrough chamber 22 are also provided for connecting the filament and theanode to external power supplies (not shown) and for connecting theworkpiece to a periodic voltage power supply 31.

Using the above-described apparatus, a workpiece is etched in accordancewith the invention, by the steps of evacuating the chamber, introducinga noble gas ambient, forming a plasma between the filament and theanode, and applying an appropriate periodic voltage between theworkpiece and the plasma, as is now explained in greater detailhereinbelow.

With the various members in place, the workpiece is surrounded with anoble gas ambient which can be ionized to form a plasma. While argon istypically used, any noble gas will work. The reason a noble gas is usedis to prevent unwanted chemical reactions between the ambient and thethin film. Ambient pressures between about one-half micron and severalhundred microns have been found to be useful. As an exception to theordinary practice of using ambients comprised solely of noble gases, ithas been found that the etching rate of oxidizable metals, such astitanium, for example, can be controlled by "bleeding" oxygen into theambient. Typically the partial pressure of the oxygen is about 1 to 5percent that of the noble gas. The oxide layer which forms on the metaletches more slowly than the pure metal, thus slowing the etching rate.Other non-noble gases which react with the particular material of thethin film surface to form slowly etching compounds can be similarlyused.

A plasma 23 is formed in the ambient between the filament 20 and theanode 21 by heating the filament by a current derived from an externalpower supply and by applying a constant voltage between the filament andthe anode as provided by another external power supply. The plasma isformed by collisions between electrons emitted from the heated filamentand atoms of the gaseous ambient. Ring magnets 28, which establish alongitudinal magnetic field, are used to control the shape of theplasma.

Etching of the thin films 11 is produced by applying an alternatingvoltage derived from an external power source 31, between the workpieceand the filament-anode system. When the workpiece is at a negativepotential with respect to the plasma, positive ions 29 are drawn out ofthe plasma and toward the workpiece where they bombard the workpiece andproduce etching and/or cleaning of the exposed surface area.

As indicated above, the workpiece may include a dielectric material asone of the layers of the thin film. When this is so, extra care must begiven to the nature and parameters of the alternative voltage that isused, and to the details of the electrical circuit through which it isapplied, in order that no significant average voltage is built up acrossthe dielectric layer. Basically, the problem arises due to the fact thatan electron is much more mobile than an ion. This means that there is amuch greater flow of electrons to the workpiece, when the latter is at apositive potential relative to the plasma, as compared to the flow ofions when the workpiece is at a negative potential relative to theplasma. As a consequence of this disparity in the mobilities of an ionand an electron, an alternating voltage having an average value of zero,such as a simplesinusoidal wave, produces a net average voltage acrossthe dielectric film.

When it appears that the resulting voltage buildup may be dangerouslynear the dielectric breakdown voltage, one or both of the followingprecautions are advantageously taken. The first of these precautions isto include a blocking capacitor in series with the workpiece. Theblocking capacitor can take the form of a second dielectric layer 30included between the workpiece and the connection to the periodicvoltage supply as shown in FIG. 2. The inclusion in the circuit of ablocking capacitor results in a sharing of the'average voltage buildupbetween the two dielectrics. In particular, if the capacitance of thecapacitor formed by the second dielectric 30 is smaller than thecapacitance of the capacitorformed by the dielectric layer, a greaterproportion of the average voltage appears across dielectric 30.

The second precaution that can be taken for reducing the voltage buildupacross the dielectric layer is to use an alternating voltage that has alow average negative voltage. This has the effect of compensating forthe lower ion mobility.

FIG. 3 illustrates a typical voltage waveform which has a negativeaverage value and is suitable for use in connection with the invention.The periodic voltage shown comprises a train of negative pulses having apulse peak, V,,,,,,, a pulse duration, t,, and a period, T. So long asthe average voltage is less than a few hundred volts, typical dielectriclayers are not damaged. It is understood, however, that the maximumpermissible average in any instance depends upon the nature of thedielectric material and its thickness. A particularly advantageousnegative pulse voltage is obtained when the pulse peak is chosen withreference to the yield curve of the materialto be etched so that theamount of etching is maximized.

FIG. 4 illustrates a typical etching yield curve. The Y-axis indicatesthe yield of atoms etched per bombarding ion, while the V-axis indicatesthe ion energy in volts. The curve is typified by three regions. As ionenergy is increased, the curve passes through a first region of lowyield until the ion energy is increased beyond a first knee (knee 1 Thislow yield region is followed by a second region of rapidly increasingyield until a second knee (knee 2) is reached. Beyond knee 2 is an upperplateau region in which the yield continues to increase but at a muchslower rate. Thus, one can maximize the amount of etching per unit ofaverage voltage by choosing a peak voltage for the negative pulse thatis approximately equal to the voltage at a low end of the upper plateauof the yield curve, i.e., near knee 2.

In order to minimize the output voltage requirements of the periodicvoltage supply 31, the impedance of the workpiece is advantageously madesmall. Since this impedance is primarily capacitive when a dielectriclayer is present, it has been found advantageous to use a high frequencyvoltage source. In practice, it has been found that the range offrequencies between kilocycles and 10 megacycles is satisfactory.

An idea of the relative magnitudes of the parameters appropriate for usein accordance with this embodiment of the invention may be obtained byconsideration of one set of parameters used in a typical operation.

In one example, a 2,000 angstrom thickness was removed from a film ofSiO disposed on a one inch diameter slice of silicon in ten minutes. Theambient used in this example was argon at a pressure of 10 microns. Thevoltage applied between the filament and the anode was 50 volts. A 0.02microfarad blocking capacitor was placed in series with the workpiece,and the waveform applied between the capacitor and ground (the workpiecebeing between the two) was a negative pulse train having a peak voltageof approximately 1 ,500 volts at frequency of I50 kilocycles per second.

It is understood that the example described above is merely intended tobe illustrative. Many other combinations of parameters within thepreviously described ranges have been successfully employed; the exactcombination being dependent upon the particular requirements of eachcase.

FIG. 5 is a schematic illustration of an alternative apparatus which canbe used for the invention. The apparatus differs from that shown in FIG.2 chiefly in that the filament and the anode of FIG. 2 have beeneliminated and in that a grounded metal electrode 40 has been added. Inthis apparatus, the workpiece is supported on an insulating base 42 in avacuum chamber 22. A quartz cylinder 24 surrounding the workpiece isalso supported by the insulator, and a grounded metal electrode isplaced across the opposite end of the cylinder. A ring magnet 41 isplaced around the cylinder 24 above the workpiece.

In this apparatus, a plasma 23 is formed by collisions between excitedelectrons and the ambient gas atoms rather than by collisions involvingfilament-emitted electrons, as in the apparatus of FIG. 2. When the gasis at a sufficiently high pressure, the application of a periodicvoltage between electrode 40 and the workpiece causes sufficientmovement of the electrons to form a plasma. However, the plasma mayadvantageously be formed at a lower pressure by the addition of magrietsfor concentrating the electron movement. Thus, for example, in FIG. 5,ring magnet 41 can be used to establish a magnetic field in thedirection of the electric field. Such a magnetic field concentrates theelectron movement and the resulting plasma within the central region ofthe ring. An advantage in forming the plasma in this manner is itssimplicity. The need for a separate electron source is eliminated, and amore nearly uniform plasma is generated. Etching or cleaning can becarried out in substantially the same manner as described previously.

In the discussion thus far, cleaning and etching of a workpiece havebeen considered. As previously mentioned, an important application ofthe invention is in the formation of an atomically clean bond between aworkpiece surface and another material.

The present invention is particularly suited to dealing with the problemof unwanted surface layers and is especially useful where the workpiececontains one or more dielectric layers. In accordance with this use ofthe invention, the surface layer is removed and prevented from reformingprior to deposition. This is done by depositing the material either atthe same time the surface layer is being removed, or immediatelyafterward, but before the workpiece is exposed to air. Known techniquessuch as, for example, sputtering or vacuum evaporation can be used todeposit the material on the cleaned surface.

FIG. 6 illustrates apparatus which can be used to atomically clean aworkpiece surface and deposit a layer of material onto it. The apparatusdiffers from that shown in FIG. 5 chiefly in that a sputtering electrode50 of the material to be deposited is placed in the quartz container 24facing the workpiece. The sputtering electrode is connected to its ownseparate power supply, which can be either alternating or directcurrent.

Cleaning of the workpiece is accomplished by ionic bombardment in themanner described above. Material is sputtered onto the workpiece fromthe sputtering electrode 50 either immediately after the cleaningprocess or, advantageously, at the same time that the cleaning is takingplace. The advantages of simultaneous cleaning and sputtering are thatthere is no time for a surface film to form and that the more looselybound atoms of sputtered material are knocked off the workpiece whilethe more tightly bound atoms remain. The result is a denser and morestrongly adherent contact.

One use for this atomically clean bonding process is the formation of ametal-semiconductor contact. FIG. 7 illustrates a typical workpiececomprising a semiconductor substrate 60 such as silicon upon which thereis shown disposed a passivating film 61 such as silicon dioxide and amasking layer 62 such as photo-resist or a metal having a highsputtering threshold.

An atomically clean contact is formed by first back sputtering theworkpiece so that the silicon substrate is exposed and then sputtering acontact metal such as platinum onto the workpiece. Back sputtering canbe carried out at a reduced rate while the sputtering is taking place inorder to achieve a cleaner, more adherent bond. By either controllingthe energy with which the sputtered atoms reach the silicon or heatingthe silicon substrate (heater not shown), an atomically clean ohmiccontact or barrier layer of metal silicide may be obtained.

When it is desired to clean the workpiece and sputter the material to bedeposited in separate steps, the substrate is first cleaned by groundingthe sputtering electrode and applying a periodic voltage to theworkpiece. After the workpiece is cleaned, it is disconnected from thepower supply or grounded, and an alternating current or a direct currentnegative voltage is applied between the sputtering electrode and groundto sputter metal onto the workpiece. A separate heating step is usuallyrequired to produce a metal silicide by sintering. An advantage of thetechnique is that both the cleaning and the metal deposition take placewithout breaking vacuum in the chamber.

The heating step can be eliminated by both etching the workpiece andsputtering metal simultaneously. The sputtered metal is ionized as itpasses through the plasma and then accelerated toward the workpiece by anegative voltage. If the workpiece contains no dielectric layers, a d.c.negative voltage can be used. However, in the usual case, a protectivelayer will be present and a periodic voltage having a negative averagevalue is used.) The metal ions are thus given sufficient energy topenetrate through surface barriers both chemical and physical and intothe lattice structure of the silicon. In addition, the impact of theionsboth of metal and of the noble gas-provide sufficient energy at theinteraction region of the silicon surface that much of the sputteredmetal reacts immediately with the silicon. By properly adjusting therate of sputtering and etching, any metal which does not react can beimmediately etched away, while the silicide builds up because it is moreetch resistant than the loosely bound, unreacted metal. Thus, thisprocess has two additional advantages: first, no separate heating stepis required saving time and reducing stress in the workpiece; and,second, no separate step is required to etch excess metal from thesurface of the workpiece.

More subtle advantages also accrue from the use of this process. Certainmetals, such as zirconium, rhodium and palladium, are very difficult touse in forming metal silicides by prior art techniques. Zirconium, whendeposited in films of 200 angstroms or more, usually cracks and peelswhen sintered. Rhodium cannot generally be chemically etched to removethe unreacted metal, and palladium usually forms a multiphase structurehaving nonuniform electrical properties. However, when the process justdescribed is used, excellent silicide contacts are formed using any ofthese metals. In. the case of zirconium, sufficient metal is implantedinto the silicon to overcome whatever surface barrier prevents theformation of good contacts, while in the case of palladium a singlephase silicide is formed. The use of sputter etching simultaneously withthe formation of rhodium silicide is clearly advantageous.

These processes and their advantages will become clearer by reference toFIG. 8, which illustrates an alternative apparatus for formingatomically clean bonds or contacts. This apparatus is substantially thesame as that described previously except that a titanium cathode 81 hasbeen added and adaptations have been made so that the workpiece 84 canbe moved from a position beneath the titanium cathode to a positionbeneath the cathode of the metal to be deposited. This adaptation isaccomplished by disposing the workpiece on graphite cathode on the outercircumference of a rotatable conductive disc 83. Electrodes 50 and 81are electrically coupled to 5,000 volt d.c. power supplies, and thegraphite cathode 80 is electrically coupled to an 800 Khz oscillatorwith a zero to minus 5,000 volt peak-to-peak output, ad-

vantageously through spring contact 82 so that the potential may be kepton the workpiece during 360 of rotation.

The workpiece, with contact windows previously etched in the protectiveoxide, is placed on the graphite and positioned so that it is beneathneither of the metal cathodes. The chamber is then evacuated to apressure of 10 torr and argon is bled in at a rate sufficient to causethe pressure in the chamber to stabilize at approximately 10 microns ofargon.

. The titanium cathode is then sputtered using a potential of 5,000volts and a current density of l milliampere per square inch for about10 minutes to be certain that the titanium surface is clean and issubsequently kept sputtering throughout the process to act as a getter.After this preliminary step, the cathode of the metal to be used informing the metal silicide the contact areas, it is rotated intoposition under the depositing cathode. The energy coupled to the siliconsurface by bombardment of noble gas ions and the impact of sputtered 1metal ions is sufficient to cause a reaction between the metal and thesilicon. For depositing platinum, it was found that rf voltages between3,000 volts and 4,500 volts, and preferably between 4,000 and 4,300volts, produced excellent platinum silicide contacts. Voltages below3,000 volts result merely in the deposition of a platinum film on thesilicon; and voltages above 4,500 volts produce a coarse platinumsilicide contact. With these voltages, any unreacted metal whichdeposits on the slice is immediately sputtered off. The intermetalliccompound formed by the metal and silicon is also removed by the backsputtering but since a two-to-one increase in volume occurs during itsformation, a net gain results at a rate of approximately 35A per minute.

After an elapsed time commensurate with the thickness of metal silicidedesired, usually in the order of 10 minutes, the d.c. potential isremoved from the metal cathode and the slice is back sputtered for anadditional 30 seconds to ensure a clean oxide surface. The slice is thenpositioned under the titanium for minutes and then the platinum for 15minutes to build up a titanium-platinum overlay. The workpiece is thenremoved from the chamber, and the remaining steps in the standardTi-Pt-Au overlay beam lead process followed, if desired.

An X-ray analysis of platinum silicide contacts formed in this mannershows it to be of the same phase and of the same preferred crystalorientation as platinum silicide formed by the standard sinteringprocess. In addition, X-ray measurements of the stresses in the siliconshow that they are only 25 percent of those developed by the standardprocess. Finally, it may be noted that Schottky barrier diodesfabricated in this manner have more nearly ideal electricalcharacteristics. The metal silicide contacts most advantageouslyproduced by the technique of this invention include TiSi, ZrSi, HfSi,NiSi and the silicides of the six platinum group metals. Sputteringvoltages in the range of l kv to 10 kv are appropriate for spontaneousformation of these materials.

It is understood that the above-described arrangements are simplyillustrative of the many possible specific embodiments which canrepresent applications of the principles of the invention. Numerous andvaried other arrangements can readily be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention.

What is claimed is: 1; A method for forming a metal silicide layer intoa silicon substrate and wherein the silicon substrate is covered with aprotective dielectric layer except for exposed regions where thesilicide layer is to be formed comprising the steps of:

cleaning the exposed regions of the silicon substrate in a low pressurenoble gas ambient by positive ion bombardment;

sputtering a metal into the exposed regions to form a metal silicidelayer and simultaneously back sputtering the silicon substrate bybombardment of noble gas ions to remove excess metal from the dielectriclayer at the same time that the metal silicide layer is being formed;and

adjusting the sputtering and back sputtering rates so that the metaldeposits only in the exposed regions thus forming the silicide layer.

2. The method of claim 1 wherein the metal is selected from the groupconsisting'of titanium, zirconium, hafnium, nickel, palladium, platinum,ruthenium, rhodium, osmium, and iridium.

37 The method of clai n 1 wh rei n the noble gas is argon.

2. The method of claim 1 wherein the metal is selected from the groupconsisting of titanium, zirconium, hafnium, nickel, palladium, platinum,ruthenium, rhodium, osmium, and iridium.
 3. The method of claim 1wherein the noble gas is argon.